Most recommended techniques for hip resurfacing and/or replacement advise the use of a jig to properly position the implant on the upper femur. Many such jigs are anchored on the femur by a reference pin, carefully placed for correct alignment. This reference pin is placed according to pre-operative planning, either manually using X-rays and plastic overlay templates, or using specialized and expensive computer software and digital patient imaging. Once this reference pin is placed during the surgery, the jig is applied, and the resurfacing or replacement operation proceeds.
A common method for reference pin placement includes placing a 115% magnified plastic overlay template over a film X-ray and adjusting the overlay template to the desired final position of the implant. 115% was chosen because a patient of average build, X-rayed in a standard manner, often appears to have been magnified to that scale on the developed X-ray film. A 115% scaled ruler on the overlay template is then used to measure the distance from the desired reference pin location to a bony landmark on the X-ray. During surgery, a regular ruler may then be used to space the corresponding reference pin location from the bony landmark on the patient's bone, and the reference pin is drilled into place.
Unfortunately, incorrect placement of the reference pin may result from inaccuracies in the recommended methods, and these errors can have deleterious effects on the outcome of the procedure. For example, there may be differences in magnification between the X-rays and the overlay templates, leading to improper placement when measurements taken from the X-rays are transferred to the actual bone at surgery. Bony landmarks are needed for either traditional plastic overlays or software-based systems. Exact localization of the bony landmarks which appear on the X-rays in two dimensions may be difficult to replicate in the three-dimensional reality of the patient's anatomy. Rotation of measuring instruments about curved surfaces or past soft tissues overlying the bony landmarks can render these known techniques unreliable. Any of these errors can result in unwanted complications, such as femoral neck failure due to malpositioning of the reference pin causing a misaligned jig and a resulting erroneously installed prosthesis.